This application claims priority to an application entitled xe2x80x9cCircuit and Method for Compensating for Non-linear Distortionxe2x80x9d filed in the Japanese Patent Office on Mar. 19, 2001 and assigned Serial No. 2001-79534, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a quadrature modulation circuit used in a radio transmitter, and in particular, to an apparatus and method for compensating for non-linear distortion generated during high-power amplification after quadrature modulation of a baseband signal.
2. Description of the Related Art
A conventional quadrature (or orthogonal) modulation circuit quadrature-modulates a baseband signal and then high-power amplifies the modulated signal. The high-power amplified modulated signal is subject to non-linear amplification in order to improve power efficiency. This is because an amplification region of an amplifier is divided into a linear region and a non-linear region and the high-power amplification is performed in the non-linear region. When amplified in the non-linear region, the amplified modulated signal suffers non-linear distortion. Thus, in order to linearize an input/output characteristic, it is necessary to compensate for distortion of the non-linearly distorted signal. A typical, conventional non-linear distortion compensation circuit includes a predistortion-type non-linear distortion compensation circuit shown in FIG. 4.
A predistortion-type non-linear distortion compensation circuit will be described with reference to FIG. 4. Referring to FIG. 4, complex baseband signals I and Q are applied to a first D/A (Digital-to-Analog) converter 2 and a second D/A converter 3 through a distortion compensation operator 1. The first and second D/A converters 2 and 3 convert received digital signals to analog signals, and provide the converted analog signals to a quadrature modulator 4. The quadrature modulator 4 quadrature-modulates received baseband signals I and Q, and provides the quadrature-modulated signals to a high-power amplifier (HPA) 5. The high-power amplifier 5 then high-power amplifies the quadrature-modulated analog signals.
A compensation data table 7 stores compensation data in the form of a table. The compensation data stored in the compensation data table 7 is determined by previously measuring a non-linear characteristic of the high-power amplifier 5 during amplification. A power calculator 6 calculates power of the baseband signals I and Q, and provides the calculated power information to the compensation data table 7. The compensation data table 7 reads compensation data corresponding to the calculated power by consulting the table according to the power of the baseband signals I and Q, and then provides the read compensation data to the distortion compensation operator 1.
In this way, the distortion compensation operator 1 previously applies an inverse distortion component for canceling the non-linear distortion generated in the high-power amplifier 4 to the received baseband signals I and Q before quadrature modulation. The signals including the inverse distortion component for removing the non-linear distortion are provided to the first and second D/A converters 2 and 3. As a result, the modulated signals high-power amplified by the high-power amplifier 5 have reduced the non-linear distortion.
As stated above, the conventional predistortion-type non-linear distortion compensation circuit compensates for non-linear distortion through the use of the data table based on the power of the baseband signals, without considering a characteristic deviation of the high-power amplifier 5 and a variation of temperature. Therefore, overall performance of the circuit may be deteriorated due to the characteristic deviation of the high-power amplifier 5 and the temperature variation.
To solve this problem, a directional combiner 8, as illustrated in FIG. 5, divides an output of the high-power amplifier 5 into two signals, and applies one of the divided signals to a quadrature demodulator 9. The quadrature demodulator 9 quadraturede-modulates the divided signal and feeds the demodulated divided signal to a compensation data operator 10. The compensation data operator 10 multiplies a coefficient, based on the feedback information, by data read from an internal compensation data table (though not shown, it is equal to the compensation data table 7 of FIG. 4). As a result, the compensation data operator 10 provides the distortion compensation operator 1 with compensated data having a high accuracy regardless of the characteristic deviation of the high-power amplifier 5 and the temperature variation.
However, since the elements 8-10 generate pseudo non-linear distortion, it is not possible to completely resolve the problem. In addition, all the elements perform a complicated digital operation, resulting in an increase in the circuit size and cast Further, the increase in the circuit size may increase power consumption, causing a reduction in a batter-run time of a mobile communication terminal using a battery as a power source.
To solve this problem, the applicant has proposed a non-linear distortion compensation circuit of FIG. 3, disclosed in Japanese patent application No. 2000-233631, the contents of which are hereby incorporated by reference. The non-linear distortion compensation circuit includes directional combiners/dividers 19 and 21, a delay circuit/phase shifter 20, an attenuator 13, a subtracter 14, a quadrature demodulator 15, a phase adjuster 22, amplitude adjusters 23 and 24, and subtracters 16 and 17. Further, the non-linear distortion compensation circuit includes a quadrature modulator 11, a carrier generator 18 and a transmission frequency control circuit 30. The transmission frequency control circuit 30 changes the frequency of a carrier signal output from the carrier generator 18 when a transmission channel is changed according to a carrier frequency setting signal.
The non-linear distortion compensation circuit interposes the directional combiner/divider 19 between the quadrature modulator 11 and a high-power amplifier 12. The directional combiner/divider 19 divides a modulated signal provided from the quadrature modulator 11 into two signals, and provides one of the divided modulated signals to the delay circuit/phase shifter 20 and provides the other divided modulated signal to the high-power amplifier 12. The delay circuit/phase shifter 20 then shifts the phase of the received signal to match it to the phase of an output signal of the attenuator 13, and then provides the phase-shifted signal to the subtracter 14.
Also, an output of the high-power amplifier 12 is divided into two signals by the directional combiner/divider 21: one of the two signals becomes an output signal and the other signal is provided to the attenuator 13. The subtracter 14 calculates a difference between the signal from the delay circuit/phase shifter 20 and the signal from the attenuator 13, and provides the calculated difference to the phase adjuster 22. That is, a non-linear distortion component calculated by the subtracter 14 is phase-adjusted through the phase adjuster 22, and then provided to the quadrature demodulator 15. Baseband non-linear distortion components output from the quadrature demodulator 15 are amplitude-adjusted to a proper level through the amplitude adjusters 23 and 24, and then provided to the subtracters 16 and 17. A non-linear distortion extractor 1A for extracting a non-linear distortion component from the non-linearly high-power amplified modulated signal includes the directional combiners/dividers 19 and 21, the delay circuit/phase shifter 20, the attenuator 13 and the subtracter 14.
The non-linear distortion compensation circuit of FIG. 3 can solve the above-stated problem, but it has the following problem. Each time a transmission channel of a radio signal is changed, i.e., each time a frequency of the carrier generator 18 is changed to other frequencies, it is necessary to readjust both phase delay of the delay circuit/phase shifter 20 and an attenuation of the attenuator 13 in the non-linear distortion extractor 1A. Specifically, a change in the transmission frequency leads to a variation in a phase delay of the delay circuit/phase shifter 20 and a gain and a non-linear distortion characteristic of the high-power amplifier 12. As a result, the modulated transmission signal is not completely removed, and the subtracter 14 cannot then extract the pure distortion component.
Therefore, it is an object of the present invention to provide a non-linear distortion compensation circuit and method for automatically setting to an optimal value a phase delay of a phase adjuster and an attenuation of an attenuator according to a transmission frequency, in a non-linear distortion extractor for extracting a non-linear distortion component generated during non-linear high-power amplification.
To achieve the above and other objects, the present invention provides a method for compensating for non-linear distortion generated during non-linear high-power amplification in a transmitter for quadrature-modulating a baseband signal, non-linearly high-power amplifying the quadrature-modulated baseband signal, extracting a non-linear distortion component from the non-linearly high-power amplified modulated signal, quadrature demodulating the extracted distortion component into a baseband distortion component, and overlapping a phase-inversed distortion component of the quadrature-demodulated baseband distortion component with the baseband signal. The method comprises shifting a phase of a signal determined by quadrature modulating the baseband signal; non-linearly high-power amplifying the quadrature-modulated signal by a high-power amplifier, and then attenuating the amplified signal by a gain of the high-power amplifier; extracting non-linear distortion generated during the non-linear high-power amplification by subtracting the phase-adjusted quadrature-modulated signal from the attenuated signal; and automatically adjusting a phase delay of the phase-adjusted signal and an attenuation of the attenuated signal according to a transmission frequency.
To achieve the above and other objects, the present invention provides a circuit for compensating for non-linear distortion generated during non-linear high-power amplification in a transmitter for quadrature-modulating a baseband signal, non-linearly high-power amplifying the quadrature-modulated baseband signal, extracting a non-linear distortion component from the non-linearly high-power amplified modulated signal, quadrature demodulating the extracted distortion component into a baseband distortion component, and overlapping a phase-inversed distortion component of the quadrature-demodulated baseband distortion component with the baseband signal. A variable phase shifter shifts a phase of a signal determined by quadrature modulating the baseband signal. A high-power amplifier non-linearly high-power amplifies the quadrature-modulated signal. A variable attenuator attenuates the amplified signal by a gain of the high-power amplifier. A subtracter extracts non-linear distortion generated during the non-linear high-power amplification by subtracting the phase-adjusted quadrature-modulated signal from an output of the attenuator. A control circuit automatically adjusts a phase delay of the variable phase shifter and an attenuation of the variable attenuator according to a transmission frequency.